Properties of a selection of objects
I have created an addon that will create a mesh cube that encompasses all selected objects. Of note is that non-render objects such as camera, lamp and lattice objects report there bounding box as a zero size cube, this can lead to a cube of zero size that is only seen in the outliner.

To use you can save this link to your addon folder or to somewhere convenient and use the Install from file button in the preferences. You will then find Create Bounding Box under the Add Mesh menu.

This will give you a single object that clearly shows the outer dimensions in the object properties as if all objects had been joined.